1. Field of the Invention
Electrodialysis ("ED") has become an accepted process and apparatus for transferring low molecular weight ions from one electrolytically conducting solution (variously referred to as the "demineralization", "diluting", "diluate", "donating", "donor", "demineralizing", "depleting" or "desalting" solution) to another (variously referred to as the "concentrating", "rceiving", "rinsing", "concentrate", "concentration", "donee", "brine" or "waste" solution). The state of the art is well described in pages 726 through 738, volume 8, Kirk-Othmer Encyclopedia of Chemical Technology, Third Edition, John Wiley and Sons, New York. Typically many, spaced, ionically-conducting, substantially hydraulically impermeable membranes are arrayed between a single pair of electrodes. The membranes are generally separated from each other by mass transport inducing structures forming electrolyte compartments (chambers, cells, spaces) therebetween. Such inter-membrane compartments are referred to herein as interior compartments, cells, chambers, or spaces. A group of interior compartments and their juxtaposed electrodes are often referred to as an electrodialysis pack. An electrodialysis stack comprises one or more such packs. The object of an ED apparatus is generally the processes taking place in the interior compartments (for example, demineralization, concentration or metathesis) induced by the passage of direct current (which may contain a substantial alternating current component) in series through the interior compartments and associated membranes. Such current is essentially ionic or electrolytic in nature, that is carried by electrically charged atoms or low molecular weight molecules passing through the interior compartments and membranes. Such charged atoms and molecules are called ions and if positively charged then cations and if negatively charged then anions.
The function of the electrodes is to convert such ionic or electrolytic current to and from the electronic current (i.e., current carried by negatively charged electrons and/or positively charged "holes") in conventional electricity distribution systems. Such conversion at a positively-charged electrode (called an anode) requires the donation of electrons by something to the electrode and/or, equivalently the acceptance by something of holes from the electrode). Such a conversion at a positively charged electrode is an oxidation process and in ED is typically the oxidation of water to oxygen gas (O.sub.2) and hydrogen ions (H.sup.+, often represented as H.sub.3 O.sup.+ and referred to as hydronium ions) and/or the oxidation of chloride (Cl.sup.-) to chlorine gas (Cl.sub.2) and/or hypochlorous acid (HOCl) and hydrogen ions. A substantial potential difference is required across the interface between the electrode and any ambient solution in order to generate such oxygen, chlorine and/or hypochlorous acid, such potential difference being sufficient in principle to oxidize many otherwise appropriate electrode materials. For example copper, iron, tin, zinc, aluminum and steel are generally rapidly oxidized (corroded) to soluble salts or ions or to insoluble oxides and hydroxides. Such corrosion can often be substantially quantitative that is, the conversion from ionic to electronic current involving principally oxidation of the electrode material itself rather than oxidation of water to oxygen or chloride to chlorine and/or hypochlorous acid. The conversion from ionic to electronic current is generally referred to as electrolysis and the materials formed by such conversion (for example, oxygen, chlorine or oxidation products of the electrode material itself) as electrolysis products or products of electrolysis. Because of such electrolysis products, electrodes are typically positioned in compartments (commonly referred to as electrode compartments, cells, chambers or spaces or as anolyte or anode compartments etc. if the electrode has always a positive polarity or as catholyte or cathode compartments etc. if the electrode has always a negative polarity), means being provided in said compartments to permit the electrodes to be flushed with appropriate electrolyte solutions to carry away the products of electrolysis. The composition of such flushing solution may often have a profound effect on the corrosion of the electrode material. For example nickel or nickel-plated steel is almost totally inert as an anode in solutions of alkali hydroxides but is rapidly corroded in chloride solutions.
Conversion from ionic to electronic current at a negatively-charged electrode (called a cathode) requires the acceptance of electrons from the electrode by something (and/or equivalently the donation by something of holes to the electrode). Such a conversion is a reduction process and in ED is typically the reduction of water to hydrogen gas (H.sub.2) and hydroxide ions (OH.sup.-, sometimes called hydroxyl ions). As mentioned above, cathodes are also generally positioned in compartments, means being provided to permit flushing with appropriate electrolyte solutions to carry away the products of electrolysis. The selection of materials for cathode is generally less difficult than for anodes and many materials are satisfactory for ED use including graphite, austenitic stainless steels (e.g., the various types 316), Incoloy 825, Hastelloy C-276, Inconel 600, titanium, niobium or zirconium depending to a certain extent on the composition of the liquid used to flush the cathode. For example chemically pure titanium may be corroded as a cathode in acid solutions. If it is necessary or desirable to utilize an acidic catholyte flush (for example to avoid precipitation of alkali insoluble substances such as calcium carbonate and magnesium hydroxide which may be formed from the hydroxide ion electrolysis product and any calcium bicarbonate or magnesium salt in the cathode flush) then Grade VII titanium or Ti Code 12 (trade name of Titanium Metal Corporation of America) will be preferred to chemically pure Ti.
ED is successfully applied in a monotonic mode in which one of the electrodes in an ED pack has always a positive polarity and the counterelectrode always a negtive polarity. In demineralization/concentration duty any specified interior compartment then always has the same function, e.g., every other interior compartment is a demineralizing compartment and the intervening interior compartments are concentrating compartments. Applications include the recovery of sodium chloride from seawater on a large scale and the demineralization of cheese whey. However, during the demineralization of surface brackish waters and some other solutions (e.g. tertiary sewage effluents) precipitates of sparingly soluble minerals (such as calcium, strontium and barium sulfate or calcium carbonate) may occur on or in some of the surfaces of some of the membranes. Further colloid materials and medium molecular weight ions (e.g., humate and fulvate anions, branched chain alkyl benzene sulfonates, polyphosphates) if present in the solution electrodialyzed may also accumulate at or in some of the surfaces of some of the membranes. Although such sparingly soluble minerals, colloid materials and medium molecular weight ions may be removed by auxiliary processes prior to ED (e.g., by water-softening, coagulation, filtration, absorption on activated carbon and the like) such processes add both cost and complexity to an ED plant. It is generally preferable to use ED in a reversing (ditonal) mode (see for example U.S. Pat. No. 2,863,813) in which the direction of passage of the direct electric current is reversed from time to time. It is found that such reversing ED (called "EDR") serves generally to remove from the membrane surfaces the sparingly soluble minerals, colloid materials, medium molecular weight ions and the like. Sub-modes of EDR are distinguishable. For example the reversal may be substantially asymmetric, e.g., the current may pass in one direction for about 14 minutes and in the opposite direction for about 1 minute. The reversal may instead be substantially symmetric e.g., passing in one direction for about 15 minutes (first half cycle) and in the opposite direction for about the same length of time (second half-cycle), the whole cycle being repeated continuously and indefinitely. Short cycle reversal (whether symmetric or unsymmetric) generally means cycle times of several minutes to a few hours whereas long cycle reversal generally signifies cycle times of several hours to several days. (Symmetric reversal has been reported in which the direction of current was changed every 7 days; asymmetric reversal in which the current was in one direction for about 148 hours and in the other for about 20 hours). It will be clear to those skilled in the art that in the case of ED operating in demineralization/concentration duty the functions of the interior ED compartments in a stack interchange when the direction of current changes, i.e., demineralization compartments become concentrating compartments and vice versa (see U.S. Pat. No. 2,863,813 cited above). For symmetric reversal and long cycle asymmetric reversal it is customary to interchange the hydraulic conduits manifolded to such groups of compartments so that the apparent flows of demineralizate and concentrate in such conduits outside the battery limits do not change (except for a brief period immediately following reversal). If minerals sparingly soluble at high pH's (such as calcium carbonate and magnesium hydroxide) are formed at a negative polarity electrode during ED then current reversal will often facilitate dissolution of dislogment (apparently from the H.sup.+ ions formed when the electrode has a positive polarity). In such case reversal can result in an ED process requiring no chemicals for removal of deposits from any surfaces, a feat unique among saline water desalting processes, making EDR a very convenient, desirable process.
In the case of monotonic ED, Hastelloy C-276 and austenitic stainless steels are generally convenient for cathodes and platinum (Pt) plated titanium (Ti) or niobium (Nb) for anodes.
In the case of EDR, each electrode in an ED stack will have positive polarity at least part of the stack operating time (duty cycle, working time). Except in the case of highly asymmetric EDR it is generally no longer satisfactory then to use Hastelloy C-276, austenitic stainless steels and the like as one of the stack electrodes and generally both electrodes are valve metals (typically titanium or niobium, the latter often referred to in the trade as columbium) thermally and/or electroplated with Pt.
An objective of this invention to provide processes and apparatuses for extending the useful anode life of electrodes in ED stacks whether operating in a monotonic or reversal mode.
This objective and others will become clear from the following description, drawings, examples and claims.
2. Description of the Prior Art
U.S. Pat. No. 3,453,201 (Mihara et. al.) discloses the use of a composite, reversing electrode at each end of an ED stack in long cycle reversal, wherein each composite electrode consists of interdigitated cathode and anode portions electrically disconnected and insulated from each other. The anode and cathode portions are chemically pure titanium or tantalum, the anode portions being platinum coated. Switching means energize, for example, the platinum coated anode portion of the first composite electrode and the cathode portion of the second composite electrode, the remaining portions of each composite electrode being contemporaneously disconnected and de-energized. During polarity reversal the anode portion of the first composite electrode is de-energized and disconnected, the cathode portion (not platinum coated) thereof is energized, the anode portion (platinum coated) of the second composite electrode is energized and the cathode portion of the latter is de-energized and disconnected. Thus each electrode portion, although not always maintained as an anode or as a cathode, is nevertheless never changed in its working polarity. A serious disadvantage of such a polarity reversing composite electrode system is that the disconnected and de-energized anode portion of each composite electrode is in fact exposed to a powerfully reducing environment when its adjacent cathode portion is working as a cathode, leading to reduction of the oxide and/or hydroxide protective layer on the platinum coating and loss of platinum as described above. Such interdigitated composite electrode systems do not substantially improve the useful life of the anode portions of each composite electrode at any given reversal cycle time, i.e., they do not substantially decrease loss of noble metal coating per ampere-hour as compared to systems wherein a single polarity reversing electrode of the same area is positioned in each electrode compartment.
U.S. Pat. No. 4,461,693 (Jain) discloses interdigitated composite electrodes in which the anode portions coated with noble metals and/or noble-metal-oxides are kept anodically polarized at all times without exceptions (whether or not working as anodes) at potentials sufficient to maintain the state of oxidation of the coatings. The direct-current power supply is necessarily more complicated and expensive than that ordinarily used in EDR or even that required for the Mihara composite electrodes. Jain also discloses noble-metal coated valve metal electrodes wherein the coating covers substantially less than the total working surface area of the membrane separating the electrode compartment from the interior compartments of the ED stack. Since it appears as mentioned above that a substantial fraction of the surface atoms are lost from the noble metal coatings as a direct result of current reversal, decreasing the fraction of the electrode which is anodically active apparently leads to an almost proportional decrease in loss of noble metal per ampere hour. Such fraction may not approach zero however and substantial loss of noble metal is still experienced in EDR, particularly short-cycle EDR.
U.S. Pat. Nos. 4,160,704 and 4,292,159 (Kuo et. al.) disclose a method and apparatus for in-situ reduction of the cathode over-voltage for hydrogen gas evolution in concentrated caustic at high temperatures in monotonically operated (i.e. non-reversing) membrane or diaphragm chloralkali electrolytic cells by introducing into the hot, concentrated caustic, ions of metals which have a low over-voltage for hydrogen gas evolution under such conditions. There is no disclosure concerning extending the life of the anodes in such processes and apparatuses.